[rkagerer]

First paragraph of the Overview section is borked:

Network analyzers are used mostly at high frequencies; operating frequencies can range from 1 Hz to 1.5 THz. Special types of network analyzers can also cover lower frequency ranges down to 1 Hz.

What's the first figure supposed to be?

[Junk_Collector]

Most likely 10 MHz. The reference links to Keysight.com and that's more or less industry standard for compensated directional couplers which are used in the large high performance VNAs. To go lower than that they usually use a directional bridge.